1. Field of the Invention
The present invention relates to the correction made to the chrominance information on colors (or tones) very near that of the flesh tones, included in color television signals encoded according to NTSC standard.
2. Discussion of the Related Art
In color television signals encoded according to NTSC standard, the chrominance information is transmitted in phase quadrature modulation. The amplitude of the modulation includes the luminance information Y. The phase and amplitude of the subcarrier include the chrominance information of the considered line. The frequency of the carrier is generally either 4.43 MHz, or 3.58 MHz under the standard.
The chrominance information includes two parameters usually referred to as B-Y and R-Y which, once they are extracted from the chrominance subcarrier by demodulation, enable the receiver to restore, with the luminance signal Y, the proportion of the three primary colors of the signal, i.e., red, green, blue in the encoded color.
Phase .psi. of the chrominance subcarrier indicates the color to be restored. Parameter B-Y is transmitted by a subcarrier which is not shifted with respect to the carrier, i.e., by a carrier having a phase of 0.degree.. Parameter R-Y is transmitted by a subcarrier having a phase of 90.degree.. Parameter Y is transmitted in phase band. Colors red and blue have specific phase positions .psi., for example 104.degree. and 0.degree. respectively. All the other colors are defined by a combination of the red and blue components. For example, green is at 255.degree. and the flesh tones is at 118.degree..
FIG. 1 represents the position of the primary colors (red, green, blue) and of the flesh tones in a phase diagram according to NTSC standard.
To improve color rendering, the colors are generally corrected by shifting certain phases .psi. of the chrominance demodulation vectors of the television signal. The correction is made in the phase half-plane centered on the flesh tones, i.e., between 28.degree. and 208.degree.. The primary colors blue (0.degree.) and green (225.degree.) therefore remain unchanged, which avoids modification of the color of the sky and grass. The most important correction is conventionally achieved at 118.degree..+-.45.degree., i.e., at 73.degree. and 163.degree. to correct the yellow-green and red-violet hues. The maximum correction is typically limited to 20.degree., which amounts to demodulating a chrominance subcarrier CHR having a phase .psi. of 73.degree. as though it had a phase of 93.degree. and a subcarrier CHR having a phase .psi. of 163.degree. as though it had a phase of 143.degree.. For the yellow-green and red-violet hues, this amounts to making the hues more red, to attenuate defects in the rendering of the color (or tones) corresponding to facial skin, which would otherwise make faces appear excessively green or blue. The flesh tones (118.degree.) is not modified.
The demodulation of television signals according to the NTSC standard is carried out by multiplying the chrominance subcarrier CHR, whose phase .psi. varies as a function of the color to be restored, by two vectors, at 0.degree. and 90.degree. (or 104.degree.), respectively. The demodulated parameters B-Y and R-Y which are obtained for a predetermined phase .psi. are then B-Y=Acos (.psi.) and R-Y=Acos(.psi.-90.degree.), where A is the modulation amplitude of the chrominance subcarrier CHR. In the case of demodulation by a vector at 104.degree., R-Y=Acos(.psi.-104.degree.).
Colors are generally corrected by modifying the phase of the two demodulation vectors by an angle .theta. corresponding to the correction to be made to phase .psi. of the chrominance subcarrier CHR. Thus, the chrominance subcarrier CHR is demodulated by multiplying it by vectors, at .theta. and .theta.+90.degree., respectively.
Parameters R-Y and B-Y then become R-Y=Acos((.psi.-.theta.)-90.degree. and B-Y=Acos(.psi.-.theta.).
FIG. 2 represents a diagram of a conventional demodulator of chrominance information of a television signal, according to NTSC standard.
The chrominance subcarrier CHR, having a phase .psi. which varies as a function of the color, is provided to a demodulator (DEMOD) 1 which restores parameters B-Y and R-Y. Demodulator 1 receives, as a demodulation vector, two signals f.sub.B and f.sub.R which, in the absence of correction, have a phase 0.degree. and 90.degree. (or 104.degree.), respectively. Signals f.sub.B and f.sub.R are generated by a phase-locked loop (PLL) including a voltage-controlled oscillator (VCO) 2, a resistive and capacitive network or RC network 3 and a phase comparator (COMP) 4. The VCO 2 receives a reference frequency f.sub.ref from a quartz crystal 5 and a phase error signal e provided by comparator 4. Signal Svco, delivered by the VCO 2, is provided to a constant phase shifter (HUE) 6 controlled by signal E.sub.6. The role of the phase shifter 6 will be explained further below.
The output S.sub.6 of the phase shifter 6 is provided, through an adder 7 whose role will be described below, to an RC network 3 which provides two output signals, f.sub.B and f'.sub.R, respectively. Signal f.sub.B constitutes the first demodulation vector of the subcarrier CHR. Signal f'.sub.R is provided to the phase comparator 4 which also receives the subcarrier CHR. Signal f'.sub.R also forms the second vector f.sub.R for demodulating the subcarrier CHR when the red color is at 90.degree.. When the demodulation is at 104.degree., signal f'.sub.R crosses a 14.degree.-phase shifter 8, which provides vector f.sub.R. A switch 9 is generally provided to select signal f.sub.R between the output signal of the RC network 3 and the output of the phase shifter 8. Switch 9 is controlled by a signal Eg provided by means (not shown) as a function of the detection of a demodulation vector (90.degree. or 104.degree.) of parameter R-Y.
At the beginning of each line scan, the PLL is synchronized during a portion of the television signal called a reference burst. Phase shifter 6, adder 7 and the phase comparator 4 are controlled by a binary signal BG indicating the presence of a portion of a reference burst of the line scan during which the phase of the subcarrier CHR is at 180.degree.. This portion of the scan is usually referred to as the burst gate BG. During the burst gate BG, the phase shifter 6 and adder 7 are disabled so that signal S.sub.7 at the output of adder 7 corresponds to signal Svco. In contrast, the phase comparator 4 is enabled during the burst gate BG. Thus, the VCO 2 is servo-controlled when the phase of signal f'.sub.R from the RC network 3 is at 90.degree. with respect to subcarrier CHR. When the loop is locked, signal Svco at the output of VCO 2 has, for example, a phase of 118.degree.. In practice, comparator 4 is enabled by signal BG and the phase shifter 6 and adder 7 are enabled by the complement BG of signal Svco
At the end of the burst gate BG, the phase comparator 4 is disabled and the phase shifter 6 and adder 7 are enabled. The phase shifter 6 is designed to provide, outside the reference bursts, a constant phase shift which is generally selected between +30.degree. and -30.degree., to compensate for a phase shift which is frequently present between the reference burst and the subcarrier encoding colors at the transmission. This phase shift is constant for a predetermined channel and the phase shifter 6 is disabled during the gate burst BG in order to avoid changing the phase lock. Adder 7 is designed to add, when the phase of the subcarrier ranges from 28.degree. to 208.degree., signal Svco (forced to 118.degree.), and a portion of the chrominance subcarrier CHR. The amplitude of signal S.sub.7 is maximum for a subcarrier CHR having a phase of 118.degree.. The phase shift between signals S.sub.6 and S.sub.7 is maximum for a subcarrier CHR having a phase of 73.degree. or 163.degree..
Hence, with respect to signal Svco, signal S.sub.7 that is transmitted to the RC network 3 has a reduced phase shift with respect to the phase shift of the subcarrier CHR when the phase of the latter ranges from 28.degree. to 208.degree.. The phases of signals f.sub.B and f.sub.R follow the phase rotation of signal S.sub.7 with respect to signal Svco. Therefore, the subcarrier CHR is demodulated as though the difference in phase of the subcarrier and 118.degree. were reduced. The tones of color near the flesh tones are thus restored closer to the true flesh tones of 118.degree..
The PLL maintains the output phases of the RC network at 0.degree. and 90.degree.. For a predetermined frequency, for example 3.58 MHz, the values of the RC network 3 force the incoming phase to 118.degree. during the gate burst BG. The RC network 3 is thus sized for a predetermined NTSC standard (3.58 MHz or 4.43 MHz).
A drawback of a conventional circuit as represented in FIG. 2 is that, since the subcarrier's amplitude is demodulated, the phase correction obtained by addition of vectors depends not only upon the phase information, but also upon the luminance information. Thus, color correction may vary over different luminance values.
It could be possible to avoid this drawback by limiting (saturating) the amplitude of the chrominance subcarrier CHR. However, this causes a distortion of signal CHR away from a sinusoidal wave form. Adding signal CHR to signal S.sub.6 would then provide a less accurate result; in addition, signal S.sub.7 is distorted due to the addition of the two signals which are not both sinusoidal. The distortion of the signal provided to the RC network 3 prevents signals R-Y and B-Y from corresponding to an accurate phase shift of 90.degree. because the input signal should be sinusoidal to obtain a phase shift of 90.degree. at the output of the RC network 3.
A further problem results from the manufacturing tolerances for the RC network 3. The relative manufacturing accuracy of such a network is generally approximately .+-.15%. The manufacturing tolerances of the RC network cause a shift of the 118.degree.-reference phase, which could cause the color corrections to be more detrimental than the color difference to be corrected.
Document DE-B-1 278 492 describes a circuit for correcting the color in a
standard television signal, which adds to the chrominance signal, a correction signal obtained from two demodulation signals phase-shifted by 90.degree. one with respect to the other. The phase rotation of the demodulation signals is achieved independently of the phase of the chrominance subcarrier and is thus the same for each color. This circuit uses adders and multipliers for adding to each demodulation signal a fraction of the other signal.
Document U.S. Pat. No. 4,173,770 also describes a circuit for globally adjusting colors by modifying, in a constant manner for all the colors, the phase of the chrominance subcarrier.